1. Field of the Invention
The present invention relates to a toner density controlling method and apparatus for use in an image forming apparatus such as a copying machine or a laser printer. More particularly, the invention relates to a toner density controlling apparatus which uses a two-component developer containing toner and a carrier.
2. Description of Related Art
In a copying machine, for example, image formation is achieved by exposing a uniformly charged photoreceptor surface to light for formation of an electrostatic latent image on the photoreceptor surface, then developing the electrostatic latent image into a visible toner image with toner, and transferring the toner image onto a sheet.
The development of the electrostatic latent image with the toner is achieved by allowing the toner to electrostatically adhere onto the photoreceptor surface by means of a developer unit. As the development operation is repeatedly performed by the developer unit, the amount of the toner retained in a developer vessel of the developer unit is reduced. Where a two-component developer is used, a carrier and a toner are retained in the developer vessel. In such a case, the amount of the toner is reduced during the repeated development operation, while the amount of the carrier is kept constant. Therefore, a T/D level (%) (wherein T is the amount of the toner and D is the amount of the developer (the total amount of the toner and the carrier)) indicative of the proportion of the toner to the developer is changed, resulting in a change in the image density.
To cope with this, it is a conventional practice to determine the change in the T/D level by sensing the magnetic permeability of the developer in the developer vessel by means of a toner sensor. The sensing of the magnetic permeability of the developer is based on the ground that the two-component developer contains a magnetic substance as the carrier and the change in the magnetic permeability is inversely proportional to the amount of the toner with the amount of the carrier kept constant.
Conventionally, the density of the toner in the developer vessel is kept constant by driving a toner feed motor to supply the toner from a hopper to the developer vessel in such a manner that the T/D level is kept constant. More specifically, a target output level of the toner sensor is preliminarily determined such that a desired T/D level can be maintained, and the operation of the toner feed motor is controlled so that the actual output voltage of the toner sensor reaches the target output level.
However, the output of the toner sensor changes with the change in the absolute humidity. Further, the relationship between the output voltage of the toner sensor and the T/D level varies with the input control voltage to be applied to the toner sensor. In addition, the output of the toner sensor decreases proportionally to the T/D level in a definitely linear relation only in a relatively limited range.
Therefore, the output of the toner sensor does not always decrease proportionally to the T/D level under any possible environmental conditions where the copying machine is used. Depending on the use environment of the copying machine, the relationship between the output of the toner sensor and the T/D level may involve uncertainty, so that the image density cannot properly be controlled.
Alternatively, a plurality of toner sensors may be employed, which are switched to cover extensive environmental conditions for correct determination of the T/D level. In such a case, however, the number of toner sensors is increased, so that the control operation of the toner sensors is complicated and the cost of the apparatus is increased.